The genus Erwinia is primarily a group of phytopathogenic and epiphytic enterobacteria associated with plants. During the past five years, the erwinias have become recognized also as emerging pathogens of animals including man. The slow and somewhat reluctant recognition of Erwinia spp. as human pathogens stems in part from a disciplinal insularity (the separate developments and practices of phytopathological, medical, and general bacteriology). We have hypothesized that the human clinical erwinias might arise from genetic recombination events in nature among Erwinia spp. and animal enterobacteria followed by the selection of animal-pathogenic recombinants. We have shown (a) that such genetic recombination by conjugation is possible in the laboratory (transfer of plasmids such as F'-lac and R factor determinants of antibiotic resistance); (b) that donor (Hfr) strains of Erwinia spp. can be generated, presumably by integration of the plasmids into the Erwinia chromosome; (c) the existence of an unusual sex (fertility) factor in human clinical Erwinia strains; (d) the molecular-genetic relationship of Erwinia spp. and other enterobacteria. The major thrust of our proposed research program is to study the genetics, ecology, physiology, and taxonomy of the plant- and animal-pathogenic and epiphytic erwinias with particular reference to the determinants of virulence for plants and animals. We will continue to define the genetic systems (including transduction and gene-linkage), attempt to transfer the genetic determinants of pathogenicity, and study the animal and plant enterobacteria comparatively (phage-typing, molecular genetics, computer taxonomy).